Method of centerless ground finishing of feedthrough pins for an implantable medical device

ABSTRACT

A centerless grinding method of finishing feedthrough pins and corresponding devices for use in implantable medical devices and for components such as batteries in implantable medical devices is disclosed. The method provides certain advantages, including the elimination of longitudinal anomalies in drawn wire to thereby improve the hermeticity of implantable medical devices. In one of the preferred methods, the surface of an over-size medical grade wire having a known anomaly depth is centerless ground using an abrasive wheel and suitable coolant to a layer past which those anomalies disappear.

This application is a divisional application of U.S. patent applicationSer. No. 08/846,772 filed Apr. 30, 1997, now U.S. Pat. No. 5,871,513,issued Feb. 16, 1999, entitled "Centerless Ground Feedthrouh Pin for anImplantable Medical Device" to Taylor et al.

FIELD OF THE INVENTION

The present invention relates to a method of removing longitudinalanomalies from feedthrough pins for hermetically sealed implantablemedical devices using a centerless grinding technique, and the devicesmade by such a method.

BACKGROUND OF THE INVENTION

There are numerous applications where it is necessary to penetrate asealed container with one or more electrical leads so as to provideelectrical access to and from electrical components enclosed within. Onesuch application for which the present invention has particular but notlimited utility is in body implantable pulse generators (e.g., for thetreatment of bradycardia, tachycardia or nerve stimulation), referred togenerally herein as implantable pulse generators (IPG's). Typicaldevices of this type are formed of a metal container housing theelectrical power source components of the IPG with a lid or the likewelded to the container to close the device and provide it with ahermetic seal.

An electrical lead or pin is electrically connected to the IPG by meansof attachment to one or more feedthroughs which penetrate the containerbut maintain the hermetically sealed environment thereof. A typicalfeedthrough consists of an external metal part, or frame or ferrule,into which an insulator solid part typically formed of glass, ceramic,or glass and ceramic is sealed. Within the insulator, one or more metalleads or pins are sealed. Since the reliability of critical implantablemedical devices depend on hermetic sealing of various components, theintegrity of such seals is of paramount importance.

In many implantable devices, metals which have long term corrosionresistance and biocompatability are needed to provide years of reliableservice since maintenance or repair possibilities for the devices areextremely limited. Moreover, since such devices are sometimes lifesavingfor the patient, failures of the hermetic seal materials can havecatastrophic consequences.

Wire fabricated by drawing or forming processes often contains anomaliessuch as drawlines, cracks or seams. See FIG. 1, where an example of aprior art drawn wire having defects such as cracks and longitudinalseams is shown.

It is a common practice to incorporate wire as conductors inglass-to-metal and ceramic-to-metal seals. The wire anomalies describedabove can induce the loss of hermeticity in such seals if theorientation of such anomalies is parallel to the seal cross-section.Deep or narrow anomalies in wire are difficult to completely fill witheither glass or solder-braze alloys that typically form the types ofhermetic seals described above. Deep or narrow anomalies may also act asstress-risers, such that application of thermal or mechanical loads tothe seal can induce latent hermetic failure.

The inability of manufactures to routinely produce wire free from theanomalies described above makes it difficult to produce reliablehermetic seals. The inability of wire manufacturers to consistently meetthe surface requirements for hermetic seal applications conflictsdirectly with the opposing requirement for highly reliability componentsin implantable medical devices.

SUMMARY OF THE INVENTION

The present invention has the object of providing solutions to at leastsome of the foregoing problems existing in the prior art.

The present invention provides certain advantages, including: (a)eliminating longitudinal anomalies in drawn wire; (b) imparting apredictable, uniform finish to the surface of drawn wire; (c) reducingmanufacturing costs by reducing substantially the scrap rates forfeedthroughs not meeting hermeticity specifications; (d) increasing thedegree of hermeticity of implantable medical devices; (e) increasing thereliability of implantable medical devices by abrasive wheelcenterless-grinding, and (f) permitting feedthrough dimensions to bemore tightly controlled than has heretofore been possible.

The present invention has certain features, including: (a) providing anover-sized medical grade wire having a known anomaly depth, andcenterless grinding the surface of the wire to a depth past which thoseanomalies disappear; (b) centerless grinding of the surface of medicalgrade wire to a desired diameter and tolerance; (c) centerless grindingof the surface of medical grade wire using an abrasive wheel andsuitable coolant; (d) centerless grinding of the surface of medicalgrade wire by locating the wire work piece using a regulating wheel andappropriately controlling work piece speed during the grinding step; (e)centerless ground feedthroughs in implantable medical devices; (f)centerless ground wire in combination with various hermetic seals inimplantable medical devices; (g) centerless ground wire in combinationwith an implantable medical device having a lithium battery disposedtherewithin; (h) centerless ground wire in combination with ceramichybrid packages or aluminum electrolytic capacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art medical grade drawn wire having typicalsurface defects and anomalies thereon;

FIG. 2 shows a flow chart of the centerless grinding process of thepresent invention;

FIG. 3 shows the transverse orientation of tool marks imparted to thesurface of centerless ground wire of the present invention;

FIG. 4 shows a perspective, cut-away view of the internal components ofan implantable medical device of the present invention;

FIG. 5 shows a cross-section view of the implantable medical device ofFIG. 4;

FIG. 6 shows a cross-sectional view of one embodiment of aceramic-to-metal feedthrough of the present invention;

FIG. 7 shows a cross-sectional view of one embodiment of a multi-pinglass-to-metal feedthrough of the present invention;

FIG. 8 shows a cross-sectional view of one embodiment of a batteryfeedthrough of the present invention and the corresponding battery foran implantable medical device;

FIG. 9 shows a cross-sectional view of one embodiment of aglass-to-metal battery feedthrough of the present invention;

FIG. 10 shows a cross-sectional view of another embodiment of aglass-to-metal battery feedthrough of the present invention; and

FIG. 11 shows comparative hermeticity test results obtained withfeedthroughs of the prior art and feedthroughs of the present inventionhaving centerless ground pins therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Centerless grinding methods are described in several printedpublications, including the first three of the following references: (a)"Research Into the Mechanics of Centerless Grinding" by W. B. Rowe,published in Precision Engineering, Volume 1, Number 1, pp. 75-84, Apr.1979; (b) "Theoretical Analysis of Rounding Effect in GeneralizedCenterless Grinding" by T. Moriya, A. Kanai, and M. Miyashita, presentedat the Proceedings of a Symposium Sponsored by the ProductionEngineering Division of the American Society of Mechanical Engineers;Fall Meeting of the Minerals, Metals & Materials Society, Chicago, Ill.,Oct. 2-6, 1994; (c) "Grit, Glue--Technology Too!," Modern Machine Shop,Volume 67, Number 7, pp. 50-60, December 1994, and (d) "Development ofHermetic Microminiature Connectors" by Neilsen et al., presented at theWinter Annual Meeting, Dallas, Tex., Nov. 25-30, 1990 of the AmericanSociety of Mechanical Engineers.

FIG. 1 illustrates a prior art medical grade drawn wire 15 havingtypical defects and anomalies 5 disposed on surface 20 thereof. Defects5 typically assume the shape of cracks or seams oriented parallel to thecentral longitudinal axis 10 of wire 15, and formed on the surfacethereof. Those seams or cracks 5 typically extend varying depths belowsurface 20 of wire 15, and may lead to or cause a loss of hermeticity inan implantable medical device or battery containing feedthroughs havingpins formed from such drawn wire. Defects 5 may assume shapes orconfigurations other than longitudinally-oriented cracks or seams suchas pits, cracks, fissures and the like.

FIG. 2 shows a flow chart of one embodiment of the centerless grindingmethod of the present invention. In FIG. 2, the method of making a pinfor a feedthrough in an implantable medical device comprises the stepsof selecting over-size medical grade wire 15 stock, straightening wire15 by passing the wire through at least one rotating die, cutting thewire to form wire 15 of desired length, centerless grinding wire 15 ofdesired length to at least substantially remove anomalies or defects 5from surface 20 thereof, cutting wire 15 to form a feedthrough pin 25having a final length and two ends, and deburring the ends. Afterdeburring, pin 25 may be cleaned in alcohol and then packaged to preventdamage thereto.

The method of the present invention may further comprise the steps ofproviding an abrasive wheel for the centerless grinding step, providinga suitable coolant for cooling the abrasive wheel during the centerlessgrinding step; providing at least one of an aluminum oxide abrasivewheel, cerium oxide abrasive wheel, boron nitride abrasive wheel,silicon carbide abrasive wheel, and diamond abrasive wheel for thecenterless grinding step, and providing one of platinum wire, stainlesssteel wire, aluminum wire, aluminum alloy wire, tantalum wire, niobiumwire and titanium wire, and wires formed of alloys, mixtures orcombinations thereof, during the selecting step.

The method of the present invention most preferably includes the step ofselecting over-size medical grade drawn wire 15 having anomalies ordefects 5 which extend a certain know depth beneath surface 20. In thecenterless grinding step of the present invention, surface 20 of wire 15is ground to a depth slightly greater than the known anomaly or defectdepth. Most preferably, surface 20 of wire 15 is ground to a depthcorresponding to a desired diameter and tolerance.

The method of the present invention is most preferably accomplishedusing an abrasive wheel or wheels in conjunction with a suitablecoolant, as for example employed by JER-NEEN MANUFACTURING COMPANY,INC., of Forest Lake, Minn. Abrasive wheels suitable for the centerlessgrinding step of the present invention include, but are not limited to,aluminum oxide, cerium oxide, boron nitride, silicon carbide and diamondwheels, and wheels having combinations of the foregoing materials.Coolants should be chosen for the appropriateness of their use withspecific wire materials. For example, soft metals such as tantalum andniobium are most suitably ground using a cutting fluid known asHALOCARBON 1.8,® CAS No. 9002-83-9 during the centerless grinding step.

FIG. 3 shows tool marks 30 on wire 15. Such marks 30 generally have anorientation transverse or substantially transverse to longitudinal axis10, and are typical of the marks formed on surface 20 of wire 15 duringa centerless grinding step of the present invention. The centerlessgrinding step removes longitudinal anomalies, cracks, seams, pits,scars, fissures and other defects 5 from surface 20 of wire 15 throughappropriate centerless application of a spinning abrasive wheel to thesurface thereof. Additionally, the centerless grinding step has beendiscovered to impart a predictable and uniform finish to surface 20 ofwire 15.

The present invention is directed especially to centerless ground pins25 in feedthroughs for lithium batteries for implantable medicaldevices, implantable pulse generators and defibrillators, implantableneurological stimulators, ceramic hybrid packages and aluminumelectrolytic capacitors. The reliability of hermetic seals utilized insuch devices has been discovered to be enhanced substantially bycenterless ground wire 15. This is because anomalies 30 induced in wire15 by centerless grinding are generally oriented transverse orsubstantially transverse to longitudinal wire axis 10, therebyredistributing interfacial stresses in a direction away from axis 10.Such redistribution of interfacial stresses has been discovered toeliminate potential pathways for helium leaks that often characterizeprior art, conventional drawn wire.

Centerless grinding and the abrasive wheels utilized therein, aredescribed in detail in the foregoing Modern Machine Shop, Rowe andMoriya papers, as well as in other printed publications known to thoseskilled in the art.

Controlled removal of material from the surface of wire 15 by centerlessgrinding is generally accomplished as follows. First, the work piece orwire is positioned using a regulating wheel, typically rubber-bonded,which sets the wire a slight predetermined angle to an abrasive grindingwheel. The regulating wheel controls the speed at which the wire rotatesduring the centerless grinding process, and brings the wire into agrinding position rapidly. The wire finds its own center as it isrotated between the regulating wheel and the abrasive grinding wheel.Out-of-round material on the wire is pushed into the grinding wheel andground away. The wire rests on a blade located between the abrasivegrinding and regulating wheels, forcing what remains into the grindingwheel at the next rotation.

A preferred centerless grinding machine for use in the method of thepresent invention is a DEDTRU centerless grinder, model no. 6/12,manufactured by UNISON CORPORATION® of Ferndale, Mich. A preferredcenterless grinding abrasive wheel for use in the method of the presentinvention is a cubic boron wheel, model no. BN180 manufactured bySUPERABRASIVES, INC. of Ferndale, Mich. A preferred feed speed for thecenterless grinding method of the present invention no more than 2-3feet per minute, with an abrasive or grinding wheel speed of about 3,500RPM. It is preferred that a spring-tempered steel blade be attached totop portion of the regulating wheel to support the wire as it is beingground by the abrasive wheel. Additionally, it is highly preferred thatstock cooling equipment not be employed in the centerless grindingmethod of the present invention, and that a custom misting apparatus beconfigured to spray a mist onto the abrasive wheel during the centerlessgrinding process as the wire is being feed past the wheel.

The method and apparatus of the present invention include glass-to-metalfeedthroughs having centerless ground pins disposed therewithin,ceramic-to-metal feedthroughs having centerless ground pins disposedtherewithin; glass/ceramic-to metal feedthroughs having centerlessground pins disposed therewithin; glass-to-ceramic feedthroughs havingcenterless ground pins disposed therewithin; hermetic feedthroughs inimplantable pulse generators and defibrillators having centerless groundpins disposed within the feedthroughs thereof; hermetic feedthroughs inimplantable power sources such as batteries and electrochemical cellshaving centerless ground pins disposed within the feedthroughs thereof;hermetic feedthroughs in aluminum electrolytic capacitors havingcenterless ground pins disposed in the feedthroughs thereof; hermeticfeedthroughs in ceramic hybrids having centerless ground pins disposedin the feedthroughs thereof, and corresponding methods of making andusing same.

FIG. 4 shows a perspective, cut-away view of the internal components ofan implantable medical device of the present invention. In FIG. 4, ageneric implantable pulse generator (or IPG) IPG 40 is shown. IPG 40includes battery section 45, hybrid electronics section 50, and acollection of feedthroughs 55, all enclosed by can, shield or container60. Conductor materials for feedthroughs 55 are selected based upontheir reported stability in contact with body fluids. Centerless groundfeedthrough pins 25 for this application may be formed from, forexample, tantalum, niobium, titanium and platinum, and alloys,combinations and mixtures thereof. Such pins 25 may be incorporated in,for example, hermetic feedthroughs utilizing ceramic-to-metal,glass-to-metal, and ceramic/glass-to-metal sealing technology. FIG. 5shows a cross-section view of the implantable medical device of FIG. 4.

FIG. 6 shows a cross-sectional view of one embodiment of a single-pinceramic-to-metal feedthrough 55 of the present invention. The presentinvention includes within its scope, however, feedthroughs 55 havingmultiple centerless ground pins 25 disposed therein. Sealing offeedthrough 55 most preferably occurs in a vacuum furnace, wheremetallized aluminum oxide insulator 65 is joined by gold braze 70 tocenterless ground pin 25 and surrounding metal body or cylinder 75.Centerless ground pin 25 is most preferably formed of tantalum, niobiumor platinum, or alloys thereof. Outer body, cylinder or ferrule 75 ismost preferably formed of a metal suitable for welding to titanium, fromwhich shield 60 of FIGS. 4 and 5 is typically formed. Gold braze 70joins insulator 65 to electrically conductive pin 25 and outer body 75.

FIG. 7 shows a cross-sectional view of one embodiment of a multi-pinglass-to-metal feedthrough of the present invention. A plurality ofcenterless ground pins 25 is shown in FIG. 7. Pins 25 are mostpreferably formed of tantalum, niobium, titanium or platinum, or alloysthereof. Pins 25 are sealingly engaged and sealing surrounded by sealingglass 80, which is selected to match the thermal expansioncharacteristics of the metal or metals from which centerless ground pins25 are formed. Ferrule, metal body or cylinder 75 sealingly surroundsand engages glass 80, and is most preferably formed of a metal suitablefor brazing or welding to a metal such as titanium, from which container60 is typically formed.

In FIG. 7, high temperature barrier glass or barrier ceramic 85sealingly engages sealing glass 80 and is selected to be compatible withthe thermal expansion characteristics of sealing glass 80. One purposeof glass or ceramic barrier 85 is to prevent contact of sealing glass 80with fixture material such as graphite during the sealing operation.Metal vapor deposits on such fixtures may result in a chemical reactionand bond with glass 80, which is undesirable.

FIG. 8 shows a cross-sectional view of one embodiment of a batteryfeedthrough 55 of the present invention and corresponding power source45 for an implantable medical device 40. Generic power source 45 may bean electrochemical cell, electrolytic capacitor, or battery in animplantable medical device 40. Centerless ground pin 25 is mostpreferably formed of molybdenum, tantalum, niobium, titanium orplatinum, or alloys thereof.

Other suitable metals for forming pin 25, depending on the particularapplication at hand and the type of feedthrough desired (i.e., pacemakerfeedthroughs, battery feedthroughs, ceramic hybrid assemblyfeedthroughs, capacitor feedthroughs, electrolytic aluminum capacitorfeedthroughs, and the like), include platinum, aluminum, aluminumalloys, stainless steel, 400-series stainless steel such as 446stainless steel (especially useful in respect of battery feedthroughapplications), and 300-series stainless steel such as 304L stainlesssteel (especially useful in respect of battery feedthroughapplications).

Referring to FIG. 7 again, pin or electrical lead wire 25 may beelectrically connected to the cathode or anode the battery 45. Sealingglass 80 in feedthrough 55 is preferably selected to be compatible withthe thermal expansion characteristics of centerless ground pin 25 andthe typically corrosive environment of the battery. For example, lithiumresistant glasses known to those skilled in the art such as TA-23,Cabal-12 or other suitably resistant glasses, are positioned around aportion of pin 25 in sealing engagement with pin 25. Battery cover 85 ismost preferably formed of "superalloys" such as MP35N, stainless steelssuch as 304L and 316L, or titanium and its alloys.

Feedthrough ferrule 75 is preferably formed of a material similar tothat employed to form battery case 85. Ferrule 75 surrounds andsealingly engages glass 80. Electrolyte fill port 90 is employed to fillbattery 45 with an appropriate amount of electrolyte fluid. Fill port 90includes a plug for sealing battery 45 once such an amount ofelectrolyte has been added thereto. Anode or cathode material isdisposed within battery case 85. The anode and cathode are separated byan ionically permeable separator 100.

Battery 45 of FIG. 8 most preferably has a lithium anode and anelectrolyte comprising a lithium salt in an organic solvent. The lithiumsalt is preferably LiCIO₄, lithium trifluorosulfonate, LiPF₆, LiCF₃ SO₃,LiBF₄, or LiAsF₆. The organic solvent is preferably propylene carbonate,glyme, diglyme, dioxolane, dimethyl sulfoxide, sulfolane, gammabutyrolactone and various mixtures thereof. The cathode material ispreferably manganese dioxide, silver vanadium oxide (Ag₂ V₄ O₁₁),vanadium oxide (V₆ O₁₃), carbon monofluoride (Cf_(x)) and variousmixtures thereof. The anode, cathode and electrolyte components ofbattery 45 are enclosed in case 85, the case preferably hermeticallysealing the components to prevent discharge of harmful gases and liquidsso that the battery may be suitable for use in applications wheresealing is critical such as in implantable medical devices. A hermeticseal provides an insulated electrical connection from the case interiorto the case exterior and preferably also maintains the hermetic seal ofthe case.

FIG. 9 shows a cross-sectional view of one embodiment of aglass-to-metal battery feedthrough of the present invention. In FIG. 9,feedthrough 55 is affixed in sealing engagement to battery case 85 orcontainer 60, most preferably by using conventional glass sealingtechniques to place and seal centerless ground pin 25 directly inaperture or opening 105 in cover 85 or container 60.

FIG. 10 shows a cross-sectional view of another embodiment of aglass-to-metal battery feedthrough of the present invention. In FIG. 10,ferrule 75 is welded in aperture or opening 105 by welds 110. Ferrule 75and centerless ground pin 25 are affixed by sealing glass 80 usingconventional sealing techniques.

FIG. 11 shows comparative hermeticity test results obtained withfeedthroughs of the prior art and feedthroughs of the present inventionhaving centerless ground pins therein. FIG. 11 shows reliabilitydifferences measured between feedthroughs incorporating conventionaldrawn pins and those incorporating centerless ground pins. Noteparticularly the differences in the number of tensile-fatigue cyclessustained by feedthroughs without hermetic loss. Feedthroughsincorporating conventional drawn tantalum wire containing longitudinalanomalies of varying depths responded repeated cycles of mechanicalstressing by beginning to lose hermeticity at 7-9 tensile-fatiguecycles. Hermeticity losses in conventional drawn wire acceleratedthereafter as the number of tensile-fatigue cycles increased. Bycomparison, feedthroughs incorporating centerless ground tantalum pinsmaintained a predetermined, specified helium leak rate after completing19-21 tensile-fatigue cycles. In fact, no tested centerless ground pinsfailed hermeticity requirements.

Those of ordinary skill will now appreciate that the method and deviceof the present invention are not limited to feedthroughs for implantablemedical devices, but extend to methods and corresponding devices forstents and IPG electrical stimulation and sensing lead wires.

Additionally, although only a few exemplary embodiments of the presentinvention have been described in detail above, those skilled in the artwill appreciate readily that many modifications are possible in theexemplary embodiments without materially departing from the novelteachings and advantages of the invention. Accordingly, all suchmodifications are intended to be included within the scope of thepresent invention as defined in the following claims.

The scope of the present invention is not limited to pacing, monitoringor sensing applications, but extends to defibrillation, neurologicalcardiac mapping and other medical and medical device applications andmethods. The scope of the present invention is not limited toapplications where a human heart is sensed, monitored, paced, ordefibrillated, but includes similar applications in other mammalians andmammalian organs.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, therefore, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing from the invention or the scope of the appendedclaims.

In the claims, means-plus-function clauses are intended to cover thestructures described herein as performing the recited function and notonly structural equivalents but also equivalent structures. Thus,although a nail and a screw may not be structural equivalents in that anail employs a cylindrical surface to secure wooden parts together,whereas a screw employs a helical surface, in the environment offastening wooden parts a nail and a screw are equivalent structures.

All patents or printed publications disclosed hereinabove are herebyincorporated by reference herein into the specification hereof, each inits respective entirety.

We claim:
 1. A glass-metal hermetic seal comprising a metal pin and asealing glass, wherein the pin is circumferentially and sealinglyengaged with the sealing glass and, prior to being assembled with theglass to form the seal, includes a surface that has been subjected to acenterless grinding process for removing defects and anomalies therefromsuch that the hermetic seal withstands at least 19-21 pin bend cycles.2. The seal of claim 1 wherein the pin is selected from the groupconsisting of titanium, molybdenum, tantalum, niobium, platinum,stainless steel, aluminum and alloys from the selected group includingcombinations thereof.
 3. The seal of claim 1 wherein the seal isselected from the group consisting of glass, ceramic and a combinationthereof.
 4. A ceramic-metal hermetic seal comprising a metal pin and asealing ceramic, wherein the pin is circumferentially and sealinglyengaged with the sealing ceramic and, prior to being assembled with theceramic to form the seal, includes a surface that has been subjected toa centerless grinding process for removing defects and anomaliestherefrom such that the hermetic seal withstands at least 19-21 pin bendcycles.
 5. The seal of claim 4 wherein the pin is selected from thegroup consisting of molybdenum, tantalum, niobium, titanium, platinum,stainless steel, aluminum and alloys from the selected group includingcombinations and mixtures thereof.
 6. The seal of claim 4 wherein theseal is selected from the group consisting of glass, ceramic, and acombination thereof.
 7. A ceramic-glass-metal hermetic seal comprising ametal pin, a sealing ceramic and glass wherein the pin iscircumferentially and sealingly engaged with the sealing ceramic and theglass, prior to being assembled with the ceramic and the glass to formthe seal, includes a surface that has been subjected to a centerlessgrinding process for removing defects and anomalies therefrom such thatthe hermetic seal withstands at least 19-21 pin bend cycles.
 8. The sealof claim 7 wherein the pin is selected from the group consisting ofmolybdenum, tantalum, niobium, platinum, stainless steel, aluminum andalloys from the selected group including, combinations and mixturesthereof.
 9. The seal of claim 7 wherein the seal is selected from thegroup consisting of glass, ceramic and a combination of ceramic andglass.